This is actually how I carried my camera on my recent world trip (About 12 cities in Europe and Asia)
I bought a rip-off Black Rapid Screw from ebay, and had that in my tripod mount. Then I had a twisty Caribeena/Harness thingy like the one used on the black rapid straps, attached to a heavy duty caribeena that I'd duct-taped together on the strap of my messenger bag. It was fantastic.
For the first couple of countries, I would tend to have my hand on the camera at all times, scared that it would detatch itself and fall onto the floor, etc. But after I got used to it, I trusted it so much, and it was brilliant.
Easy enough to take off of my bag too if I needed to. Was definitely better than carrying BOTH a black rapid and a bag, felt more comfortable and stuff, and I could carry my other lenses (and everything else you pick up whilst travelling lol) in my bag too.
TOTAL COST (Already owned the bag): $25 AU
In these pictures you can kind of see how I did it. (The last picture, my camera was not attached but you can see the caribeena. Plus im next to a TARDIS, so whatever haha)
just realized im wearing the same top in every photo. How awkward.